Thiophene chemistry and the chemical stability of the thiophene ring hold potential for use of thiophene materials in molecular-based electronics and photonics. In particular, xcex1xcex1xe2x80x2-conjugated thiophene oligomers (nTs) and polymers (polythiophenesxe2x80x94PTs have attracted great interest as semiconducting elements in organic thin-film transistors (TFTs).[1,2] To be useful in such devices and related structures, 
the organic material must support a channel of holes or electrons (p- or n-type semiconductor, respectively) created by the gate electrode bias, which switches the device xe2x80x9conxe2x80x9d. Furthermore, the charge mobility of the material must be sufficiently large to increase the source-drain on-conduction by many orders of magnitude over the xe2x80x9coffxe2x80x9d state. The density of the charge carrier in the channel is modulated by voltage applied at the gate electrode.
To date, the most noteworthy examples of this family of compounds are unsubstituted, xcex1,xcfx89- and xcex2,xcex2xe2x80x2-dialkylsubstituted nT (n=4,6), and xcex2-alkylsubstituted PT, where optimized carrier mobilities (0.1-0.6 cm2 Vxe2x88x921 sxe2x88x921) and on/off ratios ( greater than 106) approach those of amorphous silicon.[1e,2a,c,e,3] However, without exception, these systems facilitate hole injection and behave as p-type semiconductors, presumably because the thiophene electron-richness renders negative carriers susceptible to trapping by residual impurities such as oxygen[4]. Even so, increasing the number of thiophene units decreases dramatically environmental (air, light) stability and causes processing and purification difficulties.
Electron transporting (n-type) organic materials are relatively rare.[8] However, developing/understanding new n-type materials would enable applications[5] such as bipolar transistors, p-n junction diodes, and complementary circuits as well as afford better fundamental understanding of charge transport in molecular solids. The major barrier to progress however, is that most n-type organics are either environmentally sensitive, have relatively low field mobilities, lack volatility for efficient film growth, and/or are difficult to synthesize.[5e,6,7]
As indicated by the foregoing notations, these and other aspects of and teachings of the prior art can be found in the following:
[1] (a) G. Horowitz, F. Kouki, A. El Kassmi, P. Valat, V. Wintgens, F. Garnier, Adv. Mater. 1999, 11, 234. (b) F. Garnier, R. Hajaoui, A. El Kassmi, G. Horowitz, L. Laigre, W. Porzio, M. Armanini, F. Provasoli, Chem. Mater. 1998, 10, 3334. (c) X. C. Li, H. Sirringhaus, F. Garnier, A. B. Holmes, S. C. Moratti, N. Feeder, W. Clegg, S. J. Teat, R. H. Friend, J. Am. Chem. Soc. 1998, 120, 2206. (d) G. Horowitz, F. Kouki, F. Garnier, Adv. Mater. 1998, 10, 382. (e) L. Antolini, G. Horowitz, F. Kouki, F. Garnier, Adv. Mater. 1998. 10, 381. (f) G. Horowitz, Adv. Mater. 1998, 10, 365.
[2] (a) W. Li, H. E. Katz, A. J. Lovinger, J. G. Laquindanum, Chem. Mater. 1999, 11, 458. (b) H. E. Katz, J. G. Laquindanum, A. J. Lovinger, Chem. Mater. 1998, 10, 633. (c) J. G. Laquindanum, H. E. Katz, A. J. Lovinger, J. Am. Chem. Soc. 1998, 120, 664. (d) T. Siegrist, C. Kloc, R. A. Laudise, H. E. Katz, R. C. Haddon, Adv. Mater. 1998, 10, 379. (e) H. E. Katz, J. Mater. Chem. 1997, 7, 369. (f) A. Dodalabapur, L. Torsi, H. E. Katz, Science 1995, 268, 270.
[3] (a) H. Sirringhaus, P. J. Brown, R. H. Friend, K. Bechgaard, B. M. W. Lengeveld-Voss, A. J. H. Spiering, R. A. J. Janssen, E. W. Meijer, P. Herving, D. M. de Leeuw, Nature 1999, 401, 685. (b) G. Barbarella, M. Zambianchi, L. Antolini, P. Ostoja, P. Maccagnani, A. Bongini, E. A. Marseglia, E. Tedesco, G. Gigli, R. Cingolani, J. Am. Chem. Soc. 1999, 121, 8920. (c) J. H. Shxc3x6n, C. Kloc, R. A. Laudise, B. Batlogg, Appl. Phys. Lett. 1998, 73, 3574.
[4] Handbook of Heterocyclic Chemistry; A. R. Katritzky Ed.; Pergamon Press: Oxford, 1983.
[5] (a) Y. Y. Lin, A. Dodabalapur, R. Sarpeshkar, Z. Bao, W. Li, K. Baldwin, V. R. Raju, H. E. Katz, Appl. Phys. Lett. 1999, 74, 2714. (b) G. Horowitz, Adv. Mater. 1998, 10, 365. (c) A. Dodalabapur, J. G. Laquindanum, H. E. Katz, Z. Bao, Appl. Phys. Lett 1996, 69, 4227. (d) N. C. Greenham, S. C. Moratti, D. D. C. Bradley, R. H. Friend, Nature 1993, 365, 628. (e) S. Sze, Semiconductor Devices Physics and Technology; Wiley: New York, 1985; p. 481.
[6] (a) C. P. Jarret, K. Pichler, R. Newbould, R. H. Friend, Synth. Met. 1996, 77, 35. (b) R. C. Haddon, J. Am. Chem. Soc. 1996, 118, 3041. (c) G. Horowitz, F. Kouki, P. Spearman, D. Fichou, C. Nogues, X. Pan, F. Garnier, Adv. Mater. 1996, 8, 242. (d) J. G. Laquindanum, H. E. Katz, A. Dodalabapur, A. J. Lovinger, J. Am. Chem. Soc. 1996, 118, 11331.
[7] The transport properties of metal/xcex1,xcfx89-dicyano-6HT/metal structures are highly metal/interface-dependent; TFT carrier signs and mobilities have not been reported: F. Demanze, A. Yassar, D. Fichou, Synthetic Metals 1999, 101 620.
[8] (a) B. Crone, A. Dodabalapur, Y. Y. Lin, R. W. Filas, Z. Bao, A. LaDuca, R. Sarpeshkar, H. E. Katz, W. Li, Nature 401, 521, (2000); (b) H. E. Katz, A. J. Lovinger, J. Johnson, et al., Nature 404, 478 (2000).
[9] (a) R. D. McCullough, Adv. Mat. 10, 93 (1998); (b) R. L. Pilston, R. D. McCullough, Synth. Met. 111, 433 (2000); (c) X. Hong, J. C. Tyson, J. S. Middlecoff, D. M. Collard, Macromolecules 32, 4232 (1999).
As shown from the above discussion, there are a considerable number of problems and deficiencies associated with the prior art relating to useful organic n-type semiconductor compounds, compositions and/or materials, including those discussed above. There is a demonstrated need for such materials, compositions, layers and/or composites for thin film deposition and related applications useful in conjunction with the fabrication of thin film transistors and related devices as can be incorporated into an integrated circuit.
Accordingly, it is an object of the present invention to provide new and useful n-type organic materials, together with one or more methods of preparation, overcoming those various shortcomings and deficiencies of the prior art.
It will be understood by those skilled in the art that one or more aspects of this invention can meet certain objectives, while one or more other aspects can meet certain other objectives. Each objective may not apply equally, in all instances, to every aspect of the present invention. As such, the following objects can be viewed in the alternative with respect to any one aspect of the present invention.
It is another object of the present invention to provide a facile, efficient synthetic method for the preparation of an n-type thiophene conductive material, such preparation resulting in high yield and purity of the desired thiophene material.
It is yet another object of the present invention to provide n-type semiconducting thiophene compounds and related materials and/or thin films which can be used in the fabrication of and in conjunction with a variety of circuitry related devices, including, but not limited to, diode, bipolar junction transistor and field-effect transistor (either junction or metal-oxide semiconductor) devices.
It is yet another object of the present invention to provide for the synthetic modification of organic semiconductive molecular solids to alter electronic behavior, in particular the use of such modified thiophenes to provide and facilitate electron transport.
Other objects, features, benefits and advantages of the present invention will be apparent from the foregoing, in light of the summary and the examples and descriptions which follow, and will be readily apparent to those skilled in the art having knowledge of various semiconducting materials, their preparation and subsequent use. Such objects, features, benefits and advantages will be apparent from the above as taken in conjunction with the accompanying examples, tables, graphs, data and all reasonable inferences to be drawn therefrom.
In accordance with one aspect of the present invention, one or more of the foregoing objects can be achieved by use of one or more of the thiophene compounds, compositions and/or materials of the type described herein, and/or with a suitable substrate material as part of a composite or a device incorporating such a composite.
In accordance with another aspect of the present invention, one or more of the preceding objects can be achieved with a method described herein, including the use of a thiophene material as an n-type semiconductor to transport electrons.
In accordance with another aspect of the present invention, one or more of the foregoing objects can be achieved with an organic thin film transistor device which includes a source electrode, a drain electrode and a semiconductor material between the electrodes, the material preferably comprising an n-type fluoroalkyl-substituted oligo- or polythiophene composition.
In accordance with yet a further aspect of the present invention, a method is provided by way of using the introduction of substituents to a semiconducting composition to alter charge conduction there through, such that a material which would otherwise be considered a p-type conductor becomes an n-type conductor through a synthetic transformation of the type described herein.
The present invention includes the first n-type thiophene semiconductor compositions and/or materials, for use with a variety of applications or devices including, but not limited to, organic TFTs. A preferred thiophene composition/material comprises xcex1,xcfx89-diperfluorohexylsexithiophene 
designated as DFH-6T (1). Fluoroalkyl functionalization of a thiophene core significantly alters the electronic, film growth, and semiconducting properties of the resulting films.
The present invention contemplates a range of fluoroalkylated oligo- and/or polythiophene compounds, compositions and/or materials. Fluoroalkylation includes various alkyl chain lengths and fluoro-substitutions thereof, such as would result in an alteration of the electronic properties of the thiophene core from p-type to n-type semiconductivity. Without limitation, reference is made to the compositions embodied by the structural formulas and variations thereof shown in or inferred by Examples 9, 14 and 15 and claim 1, below. Known synthetic procedures can be used or modified as would be known to those skilled in the art made aware of this invention to provide a variety of thiophene cores, each with the appropriate fluoroalkyl substituents. However, for purposes such as processing and subsequent device fabrication, a preferred core has about 4-7 conjugated thiophene units. Likewise, C5-C7 fluoroalkyl substitution is preferred and can be accomplished using commercially-available reagents, but various other substitutions can be achieved through synthesis of the corresponding fluoroalkyl compounds. Thiophene core substitution is, therefore, limited only by the desired n-type semiconductivity.
A TFT device with, for instance, a DFH-6T active layer operates in the n-type accumulation mode, indicating DFH-6T and other such thiophene compounds are n-type conductors. Compared to prior art materials such as DH-6T (2) and 6T (3), the new fluorinated thiophenes of this invention are significantly more chemically and thermally inert, and can be transported quantitatively into the vapor phase without decomposition. In the solid state, the inventive thiophene units have strong xcfx80xe2x80x94xcfx80 intermolecular interactions. As described below, film growth morphologies can depend on growth temperature and substrate pretreatment and/or functionalization.
This invention demonstrates that fluoroalkyl side chain functionalization of a thiophene core significantly alters the semiconducting properties of a thiophene based conjugated oligomer or polymer material such that its conductivity switches from p-type (normally observed in unsubstituted or alkyl-substituted thiophene compounds) to n-type. This effect can be used to fabricate field-effect transistor devices that form electron accumnulation layers when a positive voltage is applied to the gate electrode. It is remarkable that the charge carrier mobility of the fluoroalkyl substituted n-type compounds is similar to that of the parent alkyl-substituted p-type compounds. Accordingly, the present invention can also include a method by which high mobility n-type semiconductors can be derived from high-mobility p-type semiconductors by replacing alkyl side chains with fluoroalkyl side chains.
Fluoroalkyl substitution substantially enhances thermal stability, volatility, and electron affinity vs. the non-fluoro analogs and affords the first n-type thiophene material composition/material for use, as an example, in a TFT. As a representative material of this invention, DFH-6T film morphology is sensitive to substrate temperature and surface pretreatment, with crystallite size increasing with increasing growth temperature. UV-vis/PL and XRD studies indicate that while DFH-6T has close intermolecular xcfx80-stacking, it is not isostructural with the DH-6T analog. Since thiophene oligomers of the prior art are typically p-type, the present invention using n-type semiconductors can provide a pathway by which an all-thiophene based complementary circuit can be realized.
In light of the preceding and with reference to the following figures and examples, the present invention is directed to a field effect transistor device including (1) a gate electrode and a semiconductor material in electrical contact therewith; (2) source and drain electrodes; and (3) an n-type semiconductor material between the source and drain electrodes and in electrical contact with, the material preferrably comprising a fluoroalkyl-substituted polythiophene composition. Such n-type polythiophene compositions include those described herein and/or claimed below. In preferred embodiments of this invention such polythiophene compositions have about four to about seven conjugated thiophene moieties and/or at least one C5-C7 fluoroalkyl substituents. Regardless, such compositions can be characterized by their electron affinities and/or field-effect mobilities through procedures and techniques well known to those skilled in the art made aware of this invention. For instance, in the preferred embodiments, such n-type polythiophene compositions have an electron affinity greater than about 3.0 eV. Various other preferred embodiments have electron affinities greater than about 4.0 eV, such affinities as can be modified for a particular end use application through choice and extent of fluoroalkyl substitution. Field-effect mobilities on the order of 10xe2x88x923 cm2/Vs and greater can be obtained depending upon choice of thiophene structure/substitution, composition/material morphology and/or film growth conditions. See, for instance, Example 9, below. For example, a transistor device in accordance with this invention can be designed having a material with such an electron affinity and/or mobility, such properties as can be provided through choice of semiconductor composition and fluoroalkyl substitution, such choice and design as more fully described herein and/or as can be made to provide, more generally, such a composition/material with n-type conductivity.
As indicated elsewhere herein, electrodes of such a field effect transistor, as well as those prepared for other transistor applications, can comprise a variety of materials well known to those skilled in the art, including gold. In preferred embodiments, such electrode materials are those useful in conjunction with the n-type semiconductor compositions described herein. In particular, such materials include but are not limited to silver and aluminum, having work functions less than about 5 eV.
In accordance with this invention, various transistor applications of the prior art are described in International Publication No. WO/9954936, the entirety of which is incorporated herein by reference. In particular, and as would be understood by those skilled in the art, a transistor of this invention can be used in conjunction with an active matrix display and/or used to switch the supply of current to a light-emissive device (LED). WO/9954936 discloses a configuration in which the drain electrode of a p-type transistor acts as the hole-injecting anode of the LED. Using n-type transistors as disclosed in the present invention, it is possible to fabricate active matrix LED displays in which the transistor acts as the cathode of the LED. When a supply voltage is connected between a source electrode of the transistor and the anode of the LED and a bias is applied to the gate electrode of the transistor, current flows from the source through the n-type thiophene semiconductor of the transistor to the drain electrode. The drain also acts as the cathode of the LED so the current then flows from the drain and through the light-emissive layer of the LED to the anode, causing light emission from the emissive layer. This configuration eliminates the need for using a reactive, low-work functional metal cathode, such as calcium or magnesium, and results in better environmental device stability.
In part, as described elsewhere herein, the present invention can also include a method of using a semiconductor material for n-type conduction in a transistor device, in particular a field effect transistor device. In preferred embodiments, such a method includes (1) providing a field effect transistor device having a gate electrode and a semiconductor material in electrical contact therewith; (2) providing source and drain electrodes with an n-type semiconductor material therebetween and in electrical contact therewith, the material comprising a fluoroalkyl-substituted polythiophene composition; and (3) operating the transistor via conduction with the polythiophene composition to provide a positive source-drain current. Application of a positive voltage to the gate electrode controls current conduction between source and drain electrodes. In various preferred embodiments, as available through use of the present invention, the gate semiconductor material can include a p-type polythiophene composition. With reference to the preceding discussion, the methodology of this invention can include operation of an associated transistor device in conjunction with a light emitting diode such that current injected into the diode can be controlled with suitable application of gate voltage. Alternatively, the n-type transistor devices of this invention can be used in combination with conventional p-type devices for the fabrication of complementary logic circuits, offering advantages of lower power consumption and better stability against variation of materials parameters, compared to all p-or all n-type logic circuits. Fabrication of complementary logic circuits requires patterning of the semiconducting layer, which can be achieved by shadow mask evaporation for oligomer materials as would be understood in the art. In the case of soluble n- and p-type materials patterning can also be achieved by known direct printing techniques such as screen printing, offset printing or inkjet printing.